The present invention relates to a process for coagulating and recovering a grafted rubber compound from an aqueous latex, and more particularly to a process which utilizes mechanical shear forces in combination with a water soluble cationic surfactant such as a quaternary ammonium salt to coagulate and recover a grafted rubber compound.
Synthetic resins are prepared in a variety of ways. For some synthetic resins, it is desirable to prepare the resin using emulsion polymerization techniques in order that a desired particle size, molecular weight, or grafting reaction may be obtained. In this art, the terms latex, aqueous emulsion, and microsuspension have all been variously used to identify such emulsion polymerization techniques.
Grafted rubber compounds are typically emulsion polymerized and then coagulated to separate the polymer resin from the water phase of the emulsion. Many techniques have been used in the past to coagulate the polymer and recover it. For example, it is known in the art to add metal salts such as magnesium or aluminum sulfate to facilitate coagulation. However, the addition of those metal salts is undesirable from a product quality standpoint. Kruse et al., U.S. Pat. No. 4,284,737, issued Aug. 18, 1981, teach the use of a cationic surfactant selected from the group of pyridinium salts, amine and quaternary ammonium salts, and imidazolinium salts.
Others have used mechanical shear forces to cause coagulation. For example, Pingel et al, U.S. Pat. No. 4,299,952, issued Nov. 10, 1981, teaches shear coagulation of latexes in the presence of steam followed by recovery of the resin solids. However, this technique is limited to only certain types of latexes which are suitable for coagulation. Typically, latexes used in emulsion polymerization processes are stabilized against coagulation by the use of anionic soaps or surfactants during polymerization. Such stabilized latexes are difficult or impossible to destabilize and coagulate by the use of shear forces.
It would be desirable to have a latex with high shear stability during polymerization but low shear stability during coagulation and recovery. Such a process would permit the polymerization process to run with greater flexibility and have less precoagulation problems and reactor fouling. Yet, during the recovery portion of the process, the latex could be readily destabilized and the resin solids could be readily isolated. Accordingly, the need still exists in this art for a process which is flexible during the emulsion polymerization portion of the process and yet permits ready coagulation and recovery of grafted resin solids from the latex.